Known wireless communications schemes may involve the use of a single antenna or multiple antennas on a transmitter and/or receiver.
A multiple input, multiple output (MIMO) wireless communication system has multiple communication channels that are used between a plurality of antennas at a transmitter and a receiver. Accordingly, in a MIMO system a transmitting device will have N transmit antennas, and the receiving device will have M receive antennas. Space-time coding controls what data is transmitted from each of the N transmit antennas. A space-time encoding function at the transmitter processes data to be transmitted and creates unique information to transmit from the N transmit antennas. Each of the M receive antennas will receive signals transmitted from each of the N transmit antennas. A space-time decoding function at the receiving device will combine the information sent from the N transmit antennas to recover the data.
As will be apparent to one of ordinary skill in the art wireless communications, coverage can be improved by employing different transmit and/or receive diversity schemes.
By contrast, system capacity can be improved by using spatial multiplexing. One way to employ diversity and/or spatial multiplexing for uplink communications is to use virtual MIMO techniques—see U.S. patent application Ser. No. 10/321,999; publication #2004-0114618. In systems employing virtual MIMO, multiple mobile stations cooperatively transmit the data of a single mobile station so as to appear as a MIMO transmission. For example, two mobile stations with one antenna each can transmit one of the mobile stations data. A two antenna base station could then receive the two signals and process them using MIMO techniques.
Transmit diversity schemes such as space-time/frequency encoding can be implemented using space time/frequency transmitter diversity (STTD/SFTD). STTD/SFTD encodes the same data in different formats for transmission from the different transmit antennas. Thus, the same data is transmitted in different formats from each of the N transmit antennas. In such a system, different transmitting antennas deliver the same information content within consecutive symbol duration or sub-carriers so that time diversity may be exploited. Usually, coverage area can be improved through the use of the transmit diversity.
Receiver diversity can be achieved by using multiple receiver antennas. The objective is to capture multiple independent (or nearly independent) signals at a receiver and combine them in an optimal manner.
Spatial multiplexing increases the capacity of a MIMO link by transmitting independent data streams in the same time slot and frequency band simultaneously from each transmit antenna, and differentiating multiple data streams at the receiver using channel information about each propagation path. Accordingly, a symbol sequence is divided into a plurality of sequences and transmitted to different transmit antennas. A different symbol is used for each transmit antenna. For example, if three antennas are used at a transmitter and a receiver, the symbol sequence is split into three independent sequences and applied to the three different antennas and transmitted. The application of spatial multiplexing can enhance data throughput and therefore system capacity.
Spatial multiplexing, transmit diversity and receive diversity can improve the capacity of a wireless communication system.